With the greater emphasis on motor vehicle aerodynamics to improve fuel economy and also styling, the angle of the windshield measured from vertical has been increasing resulting in the airflow over the hood increasing and the positive pressure at the base of the windshield decreasing. While this benefits the vehicle by reducing the drag coefficient, the reduction in static pressure at those passenger compartment air inlets located at the base of the windshield results in reduced airflow into the compartment since the driving force therefor is the pressure differential between the air entry and exit. For example, it has been found that the pressure at such a located air inlet decreases about 2% for every degree of change in the windshield angle in the currently desired range from 45.degree. to 60.degree. resulting in significant incremental reductions in air inflow.
The pressure differential can be adjusted to compensate for reduced inlet pressure by locating the outlet in a high negative (subatmospheric) pressure area such as the rear region of the rear quarter panel but this can cause negative pressures to develop in the passenger compartment under some driving conditions. And this negative compartment pressure is an undesirable effect since it may cause airflow of unconditioned ambient air into the compartment through any leakage passages such as an unseated door or window seal resulting in possibly discomforting cold or hot air drafts.